1. Field of Invention
The present invention relates to a light-emitting source and, in particular, to an organic light-emitting source.
2. Related Art
Due to the rapid development of technology, flat panel display (FPD) has become the major display. Although liquid crystal display (LCD) is one of the most popular FPD technologies, however, organic electroluminescent display possesses excellent advantages such as self-emission, low power consumption, thin thickness, light weight, wide viewing angle, rapid response rate, high contrast and brightness, wide operation temperature, easily be manufactured, low cost and full color, and it has become another popular display among FPD.
Organic electroluminescent display is composed of a plurality of organic electroluminescent devices. Organic electroluminescent device utilizes the self-emissive properties of organic functional materials to achieve display and it is classified into small molecule OLED (SM-OLED) and polymer light-emitting device (PLED) according to different molecular weights of organic functional materials.
As shown in FIG. 1, a conventional bottom light-emitting type organic electroluminescent device 1 comprises a transparent substrate 11, a transparent anode 12, an organic functional layer 13 and a metal cathode 14. The transparent substrate 11 has a first surface 111 and a second surface 112, and it is made of glass. The material of the transparent anode 12 is indium tin oxide (ITO). The organic functional layer 13 is a multilayer organic film structure. The metal cathode 14 is made of aluminum, calcium or magnesium-silver alloy. The transparent anode 12 is disposed on the first surface 111 of the transparent substrate 11. The organic functional layer 13 is disposed on the transparent anode 12. The metal cathode 14 is disposed on the organic functional layer 13. The organic functional layer 13 comprises a hole-transporting layer, an organic light-emitting layer and an electron-transporting layer.
As to the organic electroluminescent device 1, refractive index n1 of the organic functional layer 13 and refractive index n2 of the transparent anode 12 are close (n1 is about 1.7 and n2 is about 1.8 to 2.0). Refractive index n1 is greater than refractive index n3 (about 1.5) of the transparent substrate 11 and refractive index n3 is greater than that of air (about 1.0). The light 15 emitted from the organic functional layer 13 is propagated out of the device from the transparent substrate 11 because the metal cathode 14 is an opaque reflective layer.
According to Snell's Law, when the light 15 goes through an interface, the product of the refractive index and the sine of the incident angle in the incident medium are equal to that in the refractive medium. When the light 15 is propagated from the transparent anode 12 into the transparent substrate 11 and the incident angle is greater than sin−1(n3/n2), the light 15 is totally reflected and is restricted to propagating within the organic functional layer 13 and the transparent anode 12 resulting in a waveguide phenomenon between the anode and the organic functional layer. If the incident angle of the light 15 is smaller than sin−1(n3/n2), the light 15 is propagated into the transparent substrate 11. When the light 15 is propagated from the transparent substrate 11 to outside and the incident angle is greater than sin−1(1/n3), the light 15 is totally reflected and is restricted within the transparent substrate 11 resulting in a substrate waveguide phenomenon. If the incident angle of the light 15 is smaller than sin−1(1/n3), the light 15 is propagated out of the device.
As the description above, only about 20% of the light is propagated out of the device and 80% of the light is propagated within substrate and anode resulting in waveguide phenomenon then is further propagated out of the device from the lateral side. Thus, the low external quantum efficiency is the drawback of organic electroluminescent device.
However, if external quantum efficiency could be enhanced, organic electroluminescent device can not only be applied to display but also to lighting or back lighting. It is therefore an important subject of the present invention to provide an organic light-emitting source with a high external quantum efficiency to solve above-mentioned problems and be applied to lighting and back lighting.